Did you know 1mm of rain falling on 1m2 of your roof provides 1 litre of water in your tank?
Using the average suburban roof of 200m2, if you receive 500mm of rain per year, then you will harvest 100,000L of rainwater (200m2 * 500mm = 100,000L). This is a conservative estimate—many houses are bigger today and some locations can collect up to 350mm+ of rain in just one month.
Bigger is often better, but calculating what tank size you need is still an interesting exercise to do. So multiply the length of your roof by its width to get its surface area (m2), then times this by your rainfall which you can discover using Australia’s Bureau of Meteorology website.
Note: Depending on water restrictions, each person living on your property will use approximately 250L per day. So you can even factor in water usage if you’re keen.